chemical physics of discharges - Argonne National Laboratory

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The effect of adding O.O* xenon to neon is seen in Fig. 5.
The residual current drops and there is a steeper slope to the exponentti
part of the curve. In addition a smaller increase is observed in the
Faraday dark space.
(5) Energy distribution of secondary electrons. From Fig. 2 (a) we
see that the energy distribution of secondary electrons is essentially
Maxwellian at the indicated position of the probe. However, as the
axial distance is increased we find a successively greater loss of
electrons whose energies exceed about 10 eV, although thd distribution
is not radically altered from Maxwellian. The addition of 0.07% xenon
results in almost complete depletion of electrons above 12 eV and there \I
are considerable deficiencies beginning only 5 eV above the space
potential.
The data are summarized in Fig. 6. The distribution curves
for the mixture were calculated from the experimental measurements
according to the method of Medicus8, and comparison plots of the ideal
Maxwellian distribution are given. The two Curves in each case were
normalized to those regions along the potential axis which gave a I
straight line on the semi-logarithmic plot. The fluctuations in the
curves for the mixture were reproducible and are similar in appearance ‘I
to the structural features that were observed by Twiddyg in the cathode
region of rare gas discharges. It is apparent that xenon effectively
reduces the higher energy secondary electrons, even at 0.07% concentration.
Twiddy’O has shown that there is a similar loss of energetic
electrons in the positive column of an argon-neon mixture.
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(6) Residual positive ion current. The ratio of residual currents
for ie m i d i + min was 5:1 with the probe facing the cathode and about
1O:l with the probe facing the anode. These ratios were constant for
all axial positions of the probe in the negative glow and in the Faraday
dark space. The ratios for the 0.07% xenon-neon mixture were the same
as for pure neon.
DISCUSSION
Since the production of ions and low energy electrons in
the negative glow is governed largely by the rate of arrival of high
energy electrons from the cathode dark space it is obvious that the
direct observation of these high energy electrons offers an important
method for interpreting the processes of ionization in the negative
glow. A possible means of making such a measurement is t o consider the
negative residual currents collected by the screened probe when it faces
the cathode. These currents could arise from several processes:
(a) Direct collection of high energy electrons.
(b) Grid emission by high energy electrons.
(c) Grid emission by bombardment of positive ions.
(d) Grid emission by metastable atom impact.
(e) Grid emission by photon impact.
Rotation of the probe to face the anode caused a 2/3 redyctio4
in the-residual current. One would not e)tpect processes (c), (d), and
(e) to depend markedly on probe orientation so that these processes 1
probably account for less than 1/3 of the total residual negative
Current when the probe faces the cathode. Since neither- the positive ‘1
ion density nor the visible photon intensity decays exponentially,
Processes (c) and (e) are similarly rejected as a major source of curred
for the anode orientation. We conclude that the major causes of the
negative residual current in the negative glow are processes (a) and (b) i
,
),
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